Circuitry for triggering a spark gap

ABSTRACT

A circuit for triggering breakdown of an arc path through a gaseous atmosphere between a pair of spark gap defining electrodes is disclosed. The spark gap includes an auxiliary triggering electrode disposed between the pair of spark gap defining electrodes. A voltage divider network is connected across the pair of spark gap defining electrode with the output of the voltage divider being applied to the triggering electrode. The voltage divider includes a series connection of a first resistive means and a second resistive means. The first resistive means includes one or more four-layer diodes which switch rapidly from a nonconductive state to a conductive state at a certain threshold voltage applied thereacross, whereby when the potential applied across the spark gaping defining electrodes reaches a certain predetermined value the four-layer diodes break into conducting to apply the triggering output potential to the trigger electrode to trigger an arc across the pair of electrodes defining the spark gap.

United States Patent [72] lnventors Jesse A. Souza, Jr.

San Jose; Robert D. Culbertson, Campbell, both of Calif. [211 App]. No.12,550 [22] Filed Feb. 19, 1970 [45] Patented Oct. 5, 1971 [73] AssigneeVarian Associates Palo Alto, Calif.

[54] CIRCUITRY FOR TRIGGERING A SPARK GAP 3 Claims, 2 Drawing Figs.

521 US. Cl 315 200, 315/208, 102/28 [51 1 Int. Cl I -l 05b 37/00, C06c3/00 [50] Field of Search 307/284, 305; 315/185, 189, 200, 203, 205,207, 208, 330, 335, 332; 102/18, 19.2, 28, 70.2, 70.2 A; 317/80 [56]References Cited UNITED STATES PATENTS 3,293,527 12/1966 .lulich 317/80X 3,320,889 5/1967 Holtz l02/28X Primary ExaminerRoy Lake AssistantExaminerLawrence J. Dahl Attorneys-Stanley 2. Cole and Gerald L. MooreABSTRACT: A circuit for triggering breakdown of an arc path through agaseous atmosphere between a pair of spark gap defining electrodes isdisclosed. The spark gap includes an auxiliary triggering electrodedisposed between the pair of spark gap defining electrodes. A voltagedivider network is connected across the pair of spark gap definingelectrode with the output of the voltage divider being applied to thetriggering electrode. The voltage divider includes a series connectionof a first resistive means and a second resistive means. The firstresistive means includes one or more four-layer diodes which switchrapidly from a nonconductive state to a conductive state at a certainthreshold voltage applied thereacross, whereby when the potentialapplied across the spark gaping defining electrodes reaches a certainpredetermined value the four-layer diodes break into conducting to applythe triggering output potential to the trigger electrode to trigger anarc across the pair of electrodes defining the spark gap.

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. CULBERTSON BY %|MJLL f Mm ATTORNEY CIRCUITRY FOR TRIGGERING A SPARKGAP DESCRIPTION OF THE PRIOR ART l-leretofore, spark gaps have includedtriggering electrodes for triggering an arc across the main spark gap ofthe device when a certain predetermined potential was applied across thespark gap. Typically, the voltage divider included a series connectionof a first resistive means and a second resistive means; being athyratron such that when a certain potential was applied across the maingap of the spark gap and also across the voltage-divider network thethyratron would be triggered to apply an output from the voltage dividerto the triggering electrode to trigger the main spark gap. One of theproblems associated with the use of thyratrons in such a voltage-dividernetwork is that they consume relatively large amounts of power and donot break down reliably at the certain predetermined threshold voltage.In many applications, such as in detonator circuits, a spark gap isplaced in series with a detonator device for detonating explosivecharges, rockets and the like and it is especially important that thespark gap be triggered at precisely a predetermined voltage as appliedacross the voltage-divider network and thus across the main electrodesof the spark gap. Therefore, a need exists for an improved spark gaptriggering circuit for triggering spark gaps at certain predeterminedthreshold voltages. The triggering threshold voltage should preferablyhave a tolerance on the order of to percent or less.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved circuit for triggering a sparkgap.

One feature of the present invention is the provision, in a spark gaptriggering circuit having a voltage-divider network, the output of whichis applied to a triggering electrode for triggering a spark gap, of theinclusion of a four-layer diode in one of the arms of the voltagedivider, such diode serving to switch from a nonconductive state to aconductive state precisely at a certain predetermined threshold voltageapplied thereacross, whereby the spark gap is triggered at a preciselypredetermined voltage applied across the gap and-the' voltage divider.

Another feature of the present invention is the same as the precedingfeature including the provision of a current limiting resistor connectedin series with a plurality of the four-layer diodes in one arm of thevoltage-divider network for limiting the current drawn through thefour-layer diodes during conduction therethrough.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the voltage-divider network includes aseries connection of a plurality of four-layer diodes with a resistivemeans connected in parallel with each of the four-layer diodes, each ofsuch parallel connected resistors having a resistance substantiallygreater than that of the other resistive arm of the voltage-dividernetwork, whereby the voltage applied across each of the four-layerdiodes is precisely established as a certain fraction of the voltageapplied across the voltage-divider network to stabilize triggering ofthe four-layer diodes. v

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS F16. 1 is a schematic circuit diagramof a detonator circuit incorporating features of the present invention,and

FIG. 2 is a schematic line diagram of a four-layer diode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a detonator circuit 1 incorporating features of the presentinvention. The detonator circuit 1 includes a triggered spark gap device2 series connected with a detonating .wire 3 imbedded in an explosivecharge 4. A source of potential 5, as of 1,350 volts, is connectedacross the triggered spark gap device'2 and detonating wire 3 via apushbutton actuating switch 6and a low-pass filter 7 consisting of aseries connection of a resistor 8 and a shunt capacity 9. A voltagedivider network 11 is connected across the triggered spark gap device 2in parallel with the spark gap device and detonating wire 3.

The triggered spark gap device 2 includes a dielectric insulativeenvelope structure 12 containing an ionizable gas atmosphere, as of drynitrogen. A pair'of spark gap defining electrodes 13 and 14 pass throughthe envelope 12 and the space between the inner tips of the electrodes13 and 14 defines the spark gap. An auxiliary triggering electrode 15 isdisposed in the gaseous atmosphere between the pair of spark gapdefining electrodes for triggering breakdown of an arc path through thegaseous atmosphere between the pair of spark gap defining electrodes 13and 14 when a certain predetermined potential is applied across the pairof spark gaps defining electrodes and the triggering electrode 15. Thetriggering potential is derived from the output of the voltage dividernetwork 1 l.

The voltage divider network 11 includes a series connection of first andsecond resistive circuit arm 16 and 17 with the output of the voltagedivider network 11 being derived from the node between the two arms 16and 17.

The first resistive arm 16 of the voltage divider network 11 includes aseries connection of a current limiting resistor 18, as of 10 k.0,connected in series with a plurality of four-layer diodes 19. A suitablefour-layer diode comprises type UF 200--C commercially available fromUnitrode Corporation of Watertown, Massachusetts. The four-layer diode19 has a maximum forward current of 1.5 amps, a forward breakdownvoltage of 200 volts and a maximum surge current of 15 amps. The turn ontime for each diode is 0.1 to 0.5 microseconds with a turnoff time ofbetween 2 to 5 microseconds.

A typical four-layer diode is a device 19, as shown in FIG; 2, andcomprises four layers of alternating P- and N-type semiconductivematerial with PN junctions formed between each of the adjacent layers.The device is characterized by regenative feedback which produces anextremely fast semiconductive switch for switching substantial currentloads in switching times less than one microsecond. Furthermore, thefour-layer diode 19 is characterized by having an extreme-, ly preciseand predictable voltage breakdown level.

Each of the fourlayer diodes 19 includes a resistor 21 of relativelyhigh resistance, as of 3.9 megaohms, connected in parallel with each ofthe diodes 19. The series connection of the relatively high valueresistance resistors 21 serves to stabilize the potential applied acrosseach of the four-layer diodes 19. Each of the resistors 21 has a valueof resistance much higher than current limiting resistor 18 and of theresistance of the other arm 17 of the voltage-divider network 11. Thesecond resistive arm 17 of the voltage divider 11 comprises a resistor22, as of 15 kt).

In operation, pushbutton 6 is depressed to apply the 1,350 volts frompower supply 5 across the voltage-divider network 11 and the triggeredspark gap device 2. The 1,350 volts is insufficient, by itself, to breakdown the arc path between spark gap defining electrodes 13 and 14.However, the voltage is sufficient to break down the chain of four-layerdiodes 19 such that substantially half of the applied voltageappearsbetween trigger electrode 15 and the closest one of the spark gapdefining electrodes 13. The spacing between the trigger electrode 15 andthe nearest spark gap defining electrode 12 is dimensioned such that theapplied voltage from the output of the voltage divider will break downthe gap between the trigger electrode and the spark gap electrode 13.When this happens, the spark gap trigger electrode 15 assumesessentially the potential of the spark gap electrode 13 and, thus, theapplied potential of 1,350 volts appears between the trigger electrode15 and the other main spark gap electrode 14. This shifts the are fromthe trigger electrode to the other spark gap electrode 14 such that thearc is shifted or triggered between the main spark gap definingelectrodes 13 and 14. When the arc shifts to the main electrodes 13 and14, the voltage drop across the triggered spark gap device 2 and thefour-layer diodes 19 drops to a very low level and the diodes becomenonconductive and the discharge is sustained between the electrode 13and 14. Current passed by the discharge in the triggered spark gapdevice 2 energizes the detonating wire 3 to detonate the explosivecharge 4.

Premature detonation of the explosive charge 4 is prevented by having aprecisely predetermined breakdown voltage determined by the seriesconnection of the four-layer diodes 19, such that the applied triggeringDC voltage must reach the predetermined voltage level, as of 1,350volts. Low-pass filter 9 prevents high-frequency transient componentsfrom actuat ing the triggered spark gap device 2.

Since many changes could be made in the above construction and manyapparently widely difierent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a circuit for triggering breakdown of an arc path through agaseous atmosphere between a pair of spark gap defining electrodes, anauxiliary triggering electrode disposed in the gaseous atmospherebetween said pair of spark gap defining electrodes, first circuitrymeans for connecting a source of potential across said pair of spark gapdefining electrodes, second circuitry means for interconnecting saidpair of spark gap defining electrodes and said auxiliary triggeringelectrode for applying a triggering potential between said auxiliaryelectrode and one of said spark gap defining electrodes to initiate anarc between said pair of spark gap defining electrodes at a preciselypredetermined potential as applied across said pair of spark gapdefining electrodes, said second circuitry means including avoltage-divider means with the output of said voltage divider beingapplied to said auxiliary triggering electrode, said voltage-dividermeans having a series connection of first and second resistive meanswith an output connection to said auxiliary electrode disposedtherebetween. THE IMPROVEMENT WHEREIN, said first resistive meansincludes a four-layer diode which switches rapidly from substantially anonconductive state to a conductive state at a certain threshold voltageapplied thereacross, whereby at the precisely predetermined potentialapplied across said pair of spark gap defining electrodes and saidvoltage divider network said four-layer diode breaks into conduction toapply an output triggering potential to said auxiliary trigger electrodeto trigger an arc across said pair of electrodes defining said sparkgap.

2. The apparatus of claim 1 wherein said first resistive means includesa series connection of a current limiting resistor and a plurality ofseries connected four-layer diodes.

3. The apparatus of claim 2 wherein said first resistive means includesa series connection of a plurality of resistors each of which isconnected in parallel across each of said fourlayer diodes and each ofwhich has a resistance substantially greater than that of said secondresistive means of said voltage divider means.

1. In a circuit for triggering breakdown of an arc path through agaseous atmosphere between a pair of spark gap defining electrodes, anauxiliary triggering electrode disposed in the gaseous atmospherebetween said pair of spark gap defining electrodes, first circuitrymeans for connecting a source of potential across said pair of spark gapdefining electrodes, second circuitry means for interconnecting saidpair of spark gap defining electrodes and said auxiliary triggeringelectrode for applying a triggering potential between said auxiliaryelectrode and one of said spark gap defining electrodes to initiate anarc between said pair of spark gap defining electrodes at a preciselypredetermined potential as applied across said pair of spark gapdefining electrodes, said second circuitry means including avoltage-divider means with the output of said volTage divider beingapplied to said auxiliary triggering electrode, said voltage-dividermeans having a series connection of first and second resistive meanswith an output connection to said auxiliary electrode disposedtherebetween, THE IMPROVEMENT WHEREIN, said first resistive meansincludes a four-layer diode which switches rapidly from substantially anonconductive state to a conductive state at a certain threshold voltageapplied thereacross, whereby at the precisely predetermined potentialapplied across said pair of spark gap defining electrodes and saidvoltage divider network said four-layer diode breaks into conduction toapply an output triggering potential to said auxiliary trigger electrodeto trigger an arc across said pair of electrodes defining said sparkgap.
 2. The apparatus of claim 1 wherein said first resistive meansincludes a series connection of a current limiting resistor and aplurality of series connected four-layer diodes.
 3. The apparatus ofclaim 2 wherein said first resistive means includes a series connectionof a plurality of resistors each of which is connected in parallelacross each of said four-layer diodes and each of which has a resistancesubstantially greater than that of said second resistive means of saidvoltage divider means.